Current transport airplanes include cargo fire detection and suppression systems. These systems typically detect a fire by detecting smoke and then suppress detected fires by replacing oxygen in a cargo compartment with an agent, such as Halon 1301. Thus, detected cargo fires are not extinguished but instead are suppressed. That is, the suppressed fires continue to burn—but at reduced temperatures.
To that end, the Code of Federal Regulations (CFR) 25.858(b) requires that transport category airplane cargo smoke detection systems “must be capable of detecting a fire at a temperature significantly below that at which the structural integrity of the airplane is substantially decreased”. While some testing has been performed regarding cargo fire compartments, effects of a cargo fire on structural integrity of an airplane has not been investigated.
For example, Federal Aviation Agency (FAA) full scale fire testing evaluated the effectiveness of Halon 1301 replacement agents and established a Minimum Performance Standard (MPS) for any agent that could be used to replace Halon 1301 in a Class C cargo compartment. The testing identified the maximum air temperature as well as a maximum time/air temperature curve for 30 minutes. This air temperature was recorded inside the cargo compartment. The FAA full scale test article was steel lined and the test protocol used temperature detection to start the fire suppression discharge (instead of using a fire detection system of the type actually installed on airplanes with Class C car go compartments, such as an optical smoke detector). Further, the FAA test data did not provide temperatures outside the compartment or on primary structure adjacent to the cargo compartment. In addition, the steel cargo liner in a 70 to 75 degree Fahrenheit chamber provided a cooler ambient environment for dissipating the heat from a cargo fire than would occur for a worst-case airplane flight from an extreme hot day environment.
Mathematical simulations, such as computational fluid dynamics (CFD) models, can be used to analyze adjacent structure or objects. However, predicted data from these mathematical models has not been correlated to data from actual fires. Therefore, aircraft designers would have to rely on many assumptions regarding effects of heat from a cargo fire in performing thermal analysis. Without validation of the assumptions, aircraft designs are conservative to err on the side of safety of flight. This approach can result in overweight design solutions and added cost in order to protect the airplane structure from the effects of a suppressed cargo fire.
Moreover, without validated assumptions, CFD simulations of a cargo fire and the subsequent heating of the adjacent structure can be prone to errors. Analytical simulations of fires can be performed using CFD codes, such as Fire Dynamic Simulator (FDS) developed by the National Institute of Standards and Testing (NIST). The FDS code was developed to model the physics of fire, which includes the heat release rate and combustion by-products for a fire based on a defined burn rate and fuel source. However, the FDS code is not specifically designed to model the thermal impact of a fire on adjacent structure. FDS conduction heat transfer capabilities are limited to one-dimensional rectangular surfaces, while an airplane structure contains many complex curvilinear surfaces. In addition, it is difficult to determine an accurate fuel burn rate versus cargo compartment location for use in a cargo fire modeled with the FDS code. This difficulty is due to complex air flow patterns, fuel variability, and effects of fire suppressant agents on the burn process and subsequent combustion product generation.
It would therefore be desirable to correlate a signature used for detecting a fire in an airplane cargo compartment to the fire's threat to structural integrity of the airplane, due to high temperatures, after the airplane's-fire suppression system has been discharged.
The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.